[time-nuts] EFC divider resistors

[Bob Camp]

Hi

A 7805 has a *rated* temperature coefficient of 2 mv / C. Most parts do way better than this. The problem is knowing if you have a good part or not. On a 5V part, your reference could be running 400 ppm/C. Since the divider has no impact on this number, it over-rides what ever you do with the divider resistors. A good voltage reference will set you back about $50 or so. 

You commonly see a couple of mv of variation in the ground pin voltage with oven current depending on exactly where the pin is terminated. Any change in the ground setup can be an issue on an OCXO with the EFC in common with the oven (which is most parts).

Bob


On Jan 31, 2014, at 5:01 PM, Bob Stewart <bob at evoria.net> wrote:

> Hi Bob,
> 
> Do you think that maybe the oven current difference when the temperature changes could be at the root of this?  To me, it seems like this started when I put the divider in.  Before that, drift might have been hidden due to the lower resolution, I suppose.  In any case, Mouser has my money now, and we'll see what happens when better quality resistors are put in.
> 
> BTW, I'm feeding the pull-up divider with +5V from a 7805C bolted to the case.  I could switch that to the +6V  VRef from the OCXO if you think that would make any difference.  The case used to be an HP 37203A.
> 
> 
> 
> 
>> ________________________________
>> From: Bob Camp <lists at rtty.us>
>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Friday, January 31, 2014 3:50 PM
>> Subject: Re: [time-nuts] EFC divider resistors
>> 
>> 
>> Hi
>> 
>> If you are going to attenuate the EFC *and* center it up around 3V you will also need a voltage reference. Even a good one has a TC. You also have oven current running in the ground pin which contributes to voltage issues (oven and EFC share a common ground). Past that, you have thermocouple issues (resistors, wires, OCXO pins are different materials) ….
>> 
>> Bob
>> 
>> 
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